1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a mask for extreme ultraviolet (EUV) lithography.
2. Discussion of Related Art
Continual improvements in optical lithography have allowed the shrinkage of semiconductor integrated circuits (IC) to produce devices with higher density and better performance. Decreasing the wavelength used for exposure improves resolution and mitigates the effects of diffraction. Deep ultraviolet (DUV) light with a wavelength of 248 or 193 nanometers (nm) is widely used for exposure through a transmissive mask fabricated from a quartz substrate. DUV light with a wavelength of 157 or 126 nm may be used for exposure through a transmissive mask made from Calcium Fluoride. However, Next Generation Lithography (NGL) will be needed to print features with a critical dimension (CD) below 70 run.
NGL includes many different types of technology, such as EUV lithography, electron projection lithography (EPL), and proximity x-ray lithography (PXL). In order to achieve and sustain volume production, each technology will have to address and resolve many technical challenges, including the following. PXL is constrained by the difficulty of writing 1X-masks. Throughput for EPL is degraded whenever a complementary pattern must be exposed since two passes are inherently required for a stencil mask. EUV masks are expensive to fabricate due to the complexity of the processing.
To a large extent, the choice of technology for NGL depends on the desired application. However, EUV lithography appears to be the technology that can best leverage the IC industry""s existing infrastructure in DUV lithography.
EUV lithography is performed at an incidence angle of about 5 degrees with light having wavelengths of 11-15 nm. The central peak is at about 13.4 nm. Since most condensed materials absorb at EUV wavelengths, a mask for EUV lithography is typically reflective. FIG. 1(a) shows a conventional EUV mask 170 with a fixed pattern including a reflective region 173 and an anti-reflective region 176.
FIG. 1(b) shows a conventional EUV mask 170 that is fabricated on a substrate 105 having a very low coefficient of thermal expansion. First, a multilayer (ML) mirror 115 is formed over the substrate 105. The ML mirror 115 is formed from alternating layers of a high index of refraction material 111, such as 2.8 nm thick Molybdenum, and a low index of refraction material 113, such as 4.1 nm Silicon. Second, an absorber layer 125 is formed over the ML mirror 115. Third, the EUV mask 170 is patterned by lithography and etching. The absorber layer 125 is removed in some areas of the EUV mask 170 to form the reflective region 173 and left intact in other areas of the EUV mask 170 to form the antireflective region 176.
An EUV mask is costly to fabricate partly because defects in the ML mirror cannot be repaired. An advanced IC may be fabricated from about 15-30 lithography layers, of which 2-6 may be considered to be critical. Each lithography layer requires 1-2 masks so mask costs for EUV lithography can quickly become prohibitive. Consequently, reducing the cost of the mask for EUV lithography will increase the competitiveness of this technology for NGL.